In applications of electronic timing circuits it is often necessary to use an inexpensive timing oscillator due to economic and size constraints. However, the necessary accuracy required by the timing circuit is greater than the precision of the inexpensive oscillator. Thus, some means must be provided to calibrate such an oscillator by use of a more precise timing reference. This reference can be used repeatedly to calibrate a large number of the low precision oscillators.
One particular application which requires such calibration is in the field of electronic detonation of explosives. In blasting operations, it is important to efficiently utilize the blasting power of the explosives. This power must be used to achieve a maximum breakage of rock into desired sizes for a given amount of explosive material. It is further becoming increasingly important to minimize the effects of blasting on nearby structures by reducing the peak-to-peak amplitude and frequency of ground vibration produced by such a blast. The principle method for achieving these objectives is to time the ignition of a plurality of the explosive units which are placed at selected locations within an area of operation. These explosive units are detonated at intervals over a short period of time. This time-spaced detonation better applies the explosive power to the breakage of rock and at the same time reduces the peak amplitude of ground vibrations. The best results are achieved when the delay ignition can be precisely timed. However, it is difficult to achieve such required accuracy with a product which is used only once and which must be moderately priced for large volume application.
Both electric and nonelectric delay blasting caps and circuits have been previously developed. The nonelectric units are typically based upon the burning time for a pyrotechnic mixture. However, such burning time has a serious limitation in its accuracy. This limitation of accuracy prevents the optimum timing of the ignition events.
Numerous electronic detonator circuits have been proposed to achieve greater accuracy over pyrotechnic devices. Examples of such circuits are shown in U.S. Pat. Nos. 4,674,047 entitled "Integrated Detonator Delay Circuits and Fire Console", 3,851,589 entitled "Electronic Delay Blaster", 4,136,617 entitled "Electronic Delay Detonator", 4,324,182 to Kirby et al. entitled "Apparatus and Method for Selectively Activating Plural Electrical Loads at Predetermined Relative Times", 4,445,435 entitled "Electronic Delay Blasting Circuit", 4,395,950 entitled "Electronic Delay Blasting Circuit", 4,328,751 entitled "Electronic Delay Blasting Circuit", 4,145,970 entitled "Electronic Detonator Cap", 4,136,617 entitled "Electronic Delay Detonator", 4,095,527 entitled "Specialized Detonator Firing Circuit", 4,730,558 entitled "Electronic Delay-Action Explosive Detonator", 4,712,477 entitled "Electronic Delay Detonator", 4,646,640 entitled "Process and Apparatus for Chronologically Staggered Ignition of Electronic Explosive Detonating Devices", 4,623,779 entitled "Timing Apparatus for Fuse", 4,632,031 entitled "Programmable Electronic Delay Fuse", 4,601,243 entitled "Method and Apparatus for Electronically Detonating Blasting Primers", 4,586,437 entitled "Electronic Delay Detonator", 4,487,125 entitled "Timing Circuit", 4,454,815 entitled "Reprogrammable Electronic Fuse", 3,851,589 entitled "Electronic Delay Blaster", 3,646,371 entitled "Integrated Timer with Nonvolatile Memory", and 3,500,746 entitled "Weapons System with an Electronic Time Fuse".
The principle problem encountered in the use of electronic delay detonators is the accuracy of the timing mechanism (clock signal, RC time constant, etc.) within the delay circuit itself. There are numerous constraints upon the use of the local oscillator within the delay circuits. Such an oscillator should not be subjected to rate variations as a result of change in pressures, which will be experienced when one unit receives the pressure wave from the ignition of another unit. Further, fluctuations in ambient temperature, humidity and pressure can cause an oscillator to vary in its rate.
It can therefore be seen that it is important to have a method for calibrating the local oscillator in a timing circuit, such as used in electronic ignition delay circuits, so that accurate time delays between detonations can be achieved while at the same time doing so in a manner that is reliable, inexpensive and rapid.